Guys, as the title suggest...why does the reserve tank overflowing? A bit of history lesson; I havent drive the car for 2 weeks and on saturday i drive it casually to fuel up and everything is normal including the temp. When i reached home i open the hood while the engine is off to inspect the battery and notices there is coolant smell and realize coolant is overflowing from the reserve tank. It was less than 1/2 small mineral water bottle. Yesterday i drive the car again. Everything is normal and the coolant does not overflow. I have been using Proton coolant all this while and changing it at about 40k km. Water pump is replaced during last timing belt change and it got another 40k km to go. Radiator cap looks good to me. So far no milky substance on both oil filler cap and radiator cap. Is it something wrong or just an isolated case?